The invention relates to a process for producing a ferritically rolled steel strip, in which liquid steel is cast in a continuous-casting machine to form a slab and, utilizing the casting heat, is conveyed through a furnace device, undergoes preliminary rolling in a preliminary rolling device and, in a final rolling device, is finishing-rolled to form the ferritic steel strip with a desired final thickness. A process of this nature is described in patent application PCT/NL97/00325, the content of which is hereby deemed to be incorporated in the present patent application. The invention also relates to a device for producing a steel strip, suitable in particular for carrying out a process according to the invention, comprising at least one continuous-casting machine for casting thin slabs, a furnace device for homogenizing a slab, which has optionally undergone preliminary reduction, and a rolling device for rolling the slab down to a strip with the desired final thickness, and a coiler device for coiling the strip. A device of this nature is also known from patent application PCT/NL97/00325.
PCT/NL97/00325 describes a completely continuous, endless or semi-endless process for producing a steel strip which has undergone at least one rolling step in the ferritic range. The strip emerging from the final rolling device is coiled onto a coiler device, which is disposed downstream of the final rolling device, at a temperature which is such that recrystallization occurs on the coil.
Surprisingly, it has been found that the process is particularly suitable for producing a steel strip with particular properties. In this process, use is made of particular aspects of the device as described in application PCT/NL97/00325. In particular, these aspects relate to the very good control and homogeneity of the temperature of the slab or of the strip both in the width direction and in the thickness direction. The temperature is also homogeneous in the longitudinal direction, since the rolling process proceeds at a steady speed and therefore no acceleration or deceleration is required during rolling, due to the continuous, semi-endless or endless options which the device offers for rolling a ferritic strip.
The temperature homogeneity as a finction of time is also better than that which can be achieved using conventional installations. In addition, the device offers the possibility of carrying out rolling in a lubricating manner on one or more rolling mill stands. Also, cooling devices are provided at various locations in the device, so that the temperature profile of the steel slab or the steel strip can be controlled particularly successfully during its passage through and emergence from the installation.
In addition, particularly when using a vacuum tundish, the chemical composition of the steel can be particularly finely matched to the desired product properties. Moreover, owing to the good level of temperature homogeneity, the device allows a ferritic range which is very broad, i.e. extends over a wide temperature range, as explained in the patent application mentioned above.
It has been found that the known process provides a steel strip with particularly good deformation properties in an embodiment which, according to the invention, is characterized in that in a completely continuous, an endless or a semi-endless process, the slab is rolled in the austenitic range in the preliminary rolling device and, after rolling in the austenitic range, is cooled to a temperature at which the steel has a substantially ferritic structure, and the strip is rolled, in the final rolling device, at speeds which substantially correspond to the speed at which it enters the final rolling device and the following thickness reduction stages, and in at least one stand of the final rolling device, the strip is ferritically rolled at a temperature of between 850xc2x0 C. and 600xc2x0 C., and, after leaving the final rolling device, is cooled rapidly to a temperature below 500xc2x0 C. in order substantially to avoid recrystallization.
The invention works on the basis that, by cooling the ferritically rolled strip rapidly after it leaves the final rolling device, no recrystallization, or little recrystallization, occurs, and at least part of the structure which has undergone deformation in the high ferritic range is maintained. The ferritically rolled steel strip obtained in this way may, furthermore, undergo a cold ferritic reduction in the manner which is known per se, for example in such a manner that the total ferritic reduction lies in the vicinity of 70 to 80%, part of which is applied in the hot ferritic state and part in the cold ferritic state. The result is a cold-rolled steel strip with a high r-value and a low xcex94r-value. By way of indication, it can be stated that the slab thickness may be approx. 70 mm and the thickness of the reduced slab at the transition from the austenitic range to the ferritic range lies in the range between 15 and 40 mm. Rapidly cooling the hot-rolled ferritic strip to a temperature below 500xc2x0 C. prevents the deformation structure from being lost as a result of recrystallization.
DE-A-19520832 also describes a process for producing a ferritically rolled steel strip starting from liquid steel cast in a continuous-casting machine. The rolled steel strip is cooled before coiling. It is not disclosed, however, that casted slabs are conveyed through a furnace, nor that the cooling is performed so rapidly that recrystallization is avoided. WO-A92/00815 discloses a comparable process in which the strip is cooled prior to entering the last rolling stand. Again there is no disclosure about rapidly cooling which results in avoiding recrystallization.
DE-A-19600990 relates to combined austenitic and subsequent ferritic rolling. Prior to the ferritic rolling the strip is cooled. A further cooling after the ferritic rolling has not been specified.
In addition to a good temperature distribution, a good distribution of the reduction in size brought about by rolling across the thickness and width of the slab or strip is also of particular importance. Therefore, it is preferable to carry out the process in such a manner that, on at least one rolling stand at which ferritic rolling is carried out, lubrication rolling is carried out, and more particularly in such a manner that, on all the rolling stands at which ferritic rolling is carried out, lubrication rolling is carried out.
A further improvement to the stress distribution and the reduction distribution through the cross section of the slab or strip is achieved by means of a process which is characterized in that, on at least one rolling mill stand of the preliminary rolling device, rolling is carried out in a lubricating manner.
Particularly good deformation properties, i.e. high r-values and low xcex94r-values, are obtained by means of an embodiment of the process which is characterized in that the steel is an IF steel. A steel of this nature makes it possible to achieve an r-value of approx. 3. It is preferable to use an IF steel of heavy analysis with a sufficiently high titanium content and a suitably matched sulphur content, so that no interstices are formed during the ferritic rolling. A strip of this nature is particularly suitable as deep-drawing steel and as a starting material for coated strip, in particular galvanized strip.
Another embodiment of the process according to the invention is characterized in that the steel is a low-carbon steel. The known process for making DWI steels makes it possible to achieve r-values in the vicinity of 1.1. In the packing steel world, an r-value of 1.2 is desirable. With the process according to the invention, it is readily possible to achieve an r-value of 1.3 or more. The background to this is that, in contrast to the traditional method of producing DWI steel, using the process according to the invention it is possible to achieve a good starting value of the texture giving rise to the desired r-value of 1.3. In this context, low-carbon steel is understood to mean a steel with a carbon concentration of between 0.01 and 0.1%, preferably between 0.01 and 0.07%.
In order to achieve the desired high cooling rate, a further embodiment of the process according to the invention is characterized in that the strip, after leaving the final rolling device, is cooled by a cooling device with a cooling capacity of more than 2 MW/m2. In order to keep the distance between final rolling device and coiling device as short as possible and to achieve a high level of flexibility in terms of cooling rate, a further embodiment of the process according to the invention is characterized in that the cooling device has a cooling capacity of more than 3 MW/m2.
Such cooling rates can be achieved by means of a process which, according to the invention, is characterized in that, in the cooling device, use is made of water which is sprayed onto the slab by coherent jets placed with a high position density.
A cooling device which allows the cooling rates which are desirable according to the invention to be achieved is described, inter alia, in the final report of an ECSC project No. 7210-EA/214, the content of which is hereby deemed to be incorporated in the present application. A significant advantage of the cooling device which is known from this report is the wide range over which cooling capacity can be regulated, the homogeneity of the cooling and the high cooling capacity per unit surface area Selecting a high cooling capacity of this nature makes it possible to achieve the desired cooling rate at the exit speeds which arise in a continuous, endless or semi-endless rolling process.
The invention is also embodied by a device for producing a steel strip, suitable in particular for carrying out a process according to the invention, comprising at least one continuous-casting machine for casting thin slabs, a furnace device for homogenizing a slab, which has optionally undergone preliminary size reduction, and a rolling device for rolling the slab down to a strip with the desired final thickness in a preliminary rolling device, and a coiler device for coiling the strip.
Surprisingly, it is now been found that the so-called close-in-coiler directly downstream of the rolling mill stand can be avoided by an embodiment of the device which is characterized in that a cooling device with a cooling capacity of at least 2 MW/m2 is placed between the final rolling mill stand of the rolling device and the coiler device.
In the past, numerous proposals have been made for devices and processes for achieving a high cooling rate of a steel strip downstream of a rolling device and upstream of a coiler device. In the case of a device as described in PCT/NL97/00325, it is possible to produce both a ferritically rolled strip which recrystallizes on the coil and an austenitically rolled strip. In addition, the device is particularly suitable for producing a ferritically rolled steel strip according to the present invention. When producing a ferritic strip which recrystallizes on the coil, it is attempted to keep the cooling of the strip after it leaves the find rolling device as low as possible and therefore to employ a coiler device which is positioned as close as possible downstream of the final rolling device (close-in-coiler). If an austenitically rolled steel strip is being produced, this strip has to be cooled before being coiled. Therefore, the close-in-coiler which has just been mentioned is not suitable for this purpose, and a second coiler device following the cooling device is desirable. If the cooling capacity of the cooling device is high, the length over which cooling is carried out is short and the close-in-coiler can be omitted, a fact which provides the additional advantage of considerable savings.
Given a high cooling capacity of this nature, the distance between the exit sides of the final rolling device and the coiler following the cooling device is so short that the fall in temperature of a ferritically rolled steel strip over this distance is also so low that it has still proven possible to coil the strip at a temperature at which recrystallization takes place on the coil.